An analog-to-digital converter (ADC) is used for converting an analog input signal to a digital output signal. Typically, the ADC may be arranged to compare the analog input signal, or a signal derived therefrom, with one or more reference levels for generating the digital output signal.
In a successive-approximation ADC (SA-ADC), each sample of the analog input signal is compared, by means of a binary-search algorithm, with a plurality of reference levels. The bits of the digital output signal are determined in the order of significance, starting with the most significant bit (MSB) and ending with the least significant bit (LSB). For an N-bit SA-ADC, N comparisons are made for each sample in order to determine the N bits of the corresponding sample of the digital output signal.
In a parallel successive-approximation ADC (PSA-ADC), which is e.g. described in U.S. Pat. No. 5,585,796, a plurality of SA-ADCs are arranged in parallel to operate in a time-interleaved manner. Thereby an overall sampling rate is obtained, which can be considerably higher than the sampling rate of each individual SA-ADC in the plurality of SA-ADCs. Furthermore, all of the plurality of SA-ADCs uses a common reference-voltage generator, e.g. in the form of a resistor string. Thereby, the mutual matching between the individual SA-ADCs, e.g. in terms of gain and offset errors, is improved compared with if each individual SA-ADC would have its own dedicated reference-voltage generator.
As time goes by, new applications emerge that may set harder requirements on ADCs, e.g. in terms of sampling rate and/or resolution. As a nonlimiting example, some video applications, wherein an ADC e.g. may be used for sampling of RGB (Red Green Blue) signals, may require a resolution of 8-10 bits, or even higher, and a sampling rate of 270 MHz, or even higher. Such requirements may be difficult to reach.
Furthermore, newer fabrication technologies, and also a desire to reduce power dissipation, tend to dictate use of a reduced supply voltage. A reduced supply voltage may in turn cause a reduced noise margin, which e.g. may make it difficult to improve the ADC resolution.
In view of the above, there is a need for an improved PSA-ADC circuitry.